Ground-based beamformed communications using mutually synchronized spatially multiplexed feeder links

ABSTRACT

Embodiments provide ground-based beamforming with mutually synchronized spatially multiplexed gateways in a wireless communications system. Some embodiments operate in context of a satellite having a focused-beam feeder antenna that communicates with multiple, geographically distributed gateway terminals (e.g., single gateway per beam), and a user antenna that provides communications with user terminals via formed user beams. The gateway terminals can communicate feeder signals that are beam-weighted and mutually phase-synchronized (e.g., according to satellite and/or loopback beacons). For example, the synchronization can enable forward uplink signals to be phase-synchronously received by the satellite, and the beam weighting can enable the forward downlink signals to spatially combine to form forward user beams. Embodiments can achieve extensive bandwidth reuse through mutually synchronized spatial multiplexing of the feeder-link communications.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International ApplicationNo. PCT/US17/57723, filed on Oct. 20, 2017, which claims the benefit ofpriority to U.S. Provisional Application No. 62/411,377, filed on Oct.21, 2016, the entire contents of each of which are incorporated byreference herein for all purposes.

FIELD

Embodiments relate generally to communications systems, and, moreparticularly, to providing ground-based beamforming with mutuallysynchronized, spatially multiplexed feeder links.

BACKGROUND

In wireless communications systems, such as satellite communicationsystems, data can be communicated from one location to another via awireless relay. For example, in a satellite communications system, datacan be communicated between gateways and user terminals via a satellite.It is generally desirable to increase capacity of the communicationssystem. Some approaches for increasing capacity involve increasingpower, but such approaches can have various limitations. For example,power increases can be limited by power budgets (e.g., practical powerlimitations of system components, etc.) and/or by regulatory constraints(e.g., maximum allowed transmission power, etc.), and increases in powercan have a disproportionately small impact on capacity (e.g., followinga logarithmic gain when operating near the Shannon limit). Some otherapproaches involve increasing bandwidth (e.g., via greater frequencyreuse, since spectrum allocations are typically fixed and limited).However, increasing bandwidth reuse typically involves increasing thenumber of beams servicing ground terminals and decreasing beam sizes.Decreased beam sizes present a number of challenges, such as increasedsize, weight, complexity, cost, etc. of the satellite and/or groundterminals; increased accuracy required for antenna pointing and attitudecontrol in the satellite; etc. Small beam sizes also present challengeswith respect to matching the provided system capacity (e.g., providingan equal share to each of the beams) to demand (often very unevenlydistributed over the user coverage area).

Some of these concerns can be addressed for certain applications usingtechniques such as on-board beamforming arrays and hardware, but suchtechniques can further increase the size, weight, cost, and complexityof the satellite. One approach to reducing the complexity on board thesatellite, while maintaining certain features of on-board beamforming,is to shift the complexity to the ground. So-called “ground-basedbeamforming” (GBBF) approaches can be effective, but implementationshave tended to focus on lower bandwidth contexts (e.g., providing a fewMHz of user link bandwidth for L-band carrier frequencies). ConventionalGBBF has a feeder bandwidth expansion problem, as the required feederlink bandwidth is a multiple of the user link bandwidth, themultiplication factor being related to the number of antenna elementsprovided by the user link array. So for example, to provide 1 GHz ofuser bandwidth (e.g., at Ka-band) with a 100-element user-linkbeamforming array may require 100 GHz of feeder link bandwidth. Thebandwidth expansion problem can frustrate practical application ofconventional GBBF to high-capacity satellite systems.

BRIEF SUMMARY

Among other things, systems and methods are described for providingground-based beamforming with mutually synchronized, spatiallymultiplexed gateways in a wireless communications system. Someembodiments operate in the context of a satellite communications systemhaving a number of geographically distributed gateway terminals incommunication with a large number of user terminals via a satellite. Thesatellite can include a focused-beam feeder antenna that communicateswith the geographically distributed gateway terminals (e.g., singlegateway per beam), and a user antenna that services user terminals inmultiple coverage areas with formed user beams. The gateway terminalscan communicate forward signals that are beam-weighted and mutuallyphase-synchronized (e.g., according to satellite and/or loopbackbeacons), such that the forward uplink signals received by the satelliteare phase-coherent. The beam weighting is such that the signals relayed(e.g., transmitted) by the satellite spatially combine to form the userbeams. Embodiments can achieve extensive frequency reuse through spatialmultiplexing of the feeder-link signals. For example, someimplementations use very narrow feeder beams (e.g., with large satelliteantenna aperture) with frequency reuse of the same band to achievespatial multiplexing. Some implementations further increase capacity ofthe communications system and/or further reduce the number of gatewaysby exploiting multiple poles per gateway, multiple frequency sub-rangesper gateway, and/or other techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is described in conjunction with the appendedfigures:

FIG. 1 shows an illustrative satellite communications system forproviding ground-based beamforming using mutually synchronized spatiallymultiplexed feeder links (MSSMFL), according to various embodiments;

FIG. 2 shows an illustrative satellite communications system forimplementing forward-link communications, according to variousembodiments;

FIG. 3 shows an illustrative satellite communications system forimplementing return-link communications, according to variousembodiments;

FIG. 4 shows a block diagram of an illustrative satellite system forproviding MSSMFL, according to various embodiments;

FIG. 5 shows a block diagram of an illustrative loopback pathway forloopback beacon signaling, according to various embodiments;

FIG. 6 shows a block diagram of a satellite beacon subsystem, accordingto various embodiments;

FIG. 7 shows an illustrative satellite communications system forproviding MSSMFL, according to various embodiments;

FIG. 8 shows another illustrative satellite communications system forproviding MSSMFL, according to various embodiments;

FIG. 9 shows a block diagram of an illustrative satellite system forproviding MSSMFL using multiple polarization orientations, according tovarious embodiments;

FIG. 10 shows a block diagram of an illustrative satellite system forproviding MSSMFL using multiple frequency sub-ranges, according tovarious embodiments;

FIG. 11 shows a flow diagram of an illustrative method for ground-basedbeamforming with MSSMFL in a satellite communications system, accordingto various embodiments;

FIG. 12 shows a plot of a feeder-link antenna pattern for anillustrative feeder reflector plotted with respect to azimuth andelevation;

FIG. 13 shows a block diagram of an illustrative calibration systemimplemented on the satellite to assist in making forward-linkmeasurements of feeder-link antenna pattern impairments;

FIG. 14 shows a block diagram of an illustrative calibration systemimplemented on the satellite to assist in making return-linkmeasurements of feeder-link antenna pattern impairments; and

FIG. 15 shows a block diagram of an illustrative feeder antenna patternimpairment compensation environment that includes an illustrativecrosstalk canceler.

In the appended figures, similar components and/or features can have thesame reference label. Further, various components of the same type canbe distinguished by following the reference label by a second label thatdistinguishes among the similar components. If only the first referencelabel is used in the specification, the description is applicable to anyone of the similar components having the same first reference labelirrespective of the second reference label.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present invention. However, onehaving ordinary skill in the art should recognize that the invention canbe practiced without these specific details. In some instances,circuits, structures, and techniques have not been shown in detail toavoid obscuring the present invention.

Embodiments described herein include novel techniques for providingground-based beamforming with mutually synchronized spatiallymultiplexed gateways in a wireless communications system (referred toherein as mutually synchronized spatially multiplexed feeder links, orMSSMFL). Some such techniques include mutually phase-synchronizing andbeam-weighting spatially multiplexed feeder-link signals in the groundsegment of the communications system. For example, in the forwarddirection, focused feeder beams can be used to receive the mutuallyphase-synchronized and beam-weighted, spatially multiplexed forwarduplink signals at a satellite (e.g., or any other suitable wirelesscommunications relay). The satellite can use an antenna array to relaythe mutually phase-synchronized signals in such a way that forms userbeams according to the beam-weighting of the signals. Because the feederbeams are focused, they can be directed to different(spatially-separated) regions, allowing feeder links to reuse the samefrequency band. This resulting frequency reuse thus avoids the bandwidthexpansion problem of conventional ground-based beamforming. Accordingly,novel types of ground-based beamforming with MSSMFL described herein canfacilitate implementation of high-throughput satellite communicationssystems, such as systems providing throughputs of 1 Terabit-per-secondor more.

Turning to FIG. 1, an illustrative satellite communications system 100is shown for providing MSSMFL, according to various embodiments. Asillustrated, the satellite communications system 100 includes asatellite 140 in communications with multiple (M) gateway terminals 130and multiple user terminals 165. The satellite 140 can be implemented asa bent-pipe (e.g., non-processed) geosynchronous (GEO) communicationssatellite. Alternatively, the satellite 140 can be implemented as anyother suitable satellite or wireless communications relay as describedherein. The gateway terminals 130 are geographically distributed andcommunicate with the satellite via focused feeder beams. For example,the gateway terminals 130 are disposed in at least two feeder beamswithout overlapping with each other (e.g., as single spot beam pergateway terminals 130, or any other suitable implementation). All Mgateway terminals 130 operate at a same carrier frequency. Note,however, that for gateways that are mutually-synchronized as describedherein, there may be brief periods of time during which the gateways donot operate at exactly the same carrier frequency due to tolerances,cycle slips, phase/frequency noise, and short terms loss ofsynchronization. Moreover, a gateway may transmit multiple carrierssimultaneously each at a different frequency. However, for transmissionof multiple carriers, any individual carrier which is beamformed by thesatellite will be transmitted at the same frequency by each gateway.Thus, gateways may be said to transmit at substantially the samefrequency without altering the scope of the present invention. The userterminals 165 are disposed in one or more user beam coverage areas 160associated with one or more formed user beams (e.g., using beamformingantenna elements on the satellite 140).

The gateway terminals 130 are in communication with other ground segmentcomponents, which can enable MSSMFL through ground-based coordination ofphase-synchronized feeder-link signals and beamformed user-link signals.As illustrated, feeder-link modems 103 can be in communication with adigital data network 101, such as the Internet, or the like. Thefeeder-link modems 103 can convert between digital data network 101traffic and data streams 105 associated with user beam coverage areas160. For example, K data streams 105 can be associated with traffic toand from K user beam coverage areas 160 (corresponding to K formed userbeams).

The feeder-link modems 103 can be coupled with forward and/or returnbeamformers 110, and the forward and/or return beamformers 110 can be incommunication with the gateway terminals 130. For example, each gatewayterminal 130 includes a communications link with the forward and/orreturn beamformers 110 via a distribution network 120, such as anInternet backhaul network, or any other suitable network. Embodiments ofthe forward and/or return beamformers 110 can apply forward and/orreturn beam weights to forward and/or return signals, respectively. Forexample, in the forward direction, the forward and/or return beamformers110 can generate beam-weighted forward signals from the K data streams105, which can be communicated to the gateway terminals 130. In thereturn direction, the forward and/or return beamformers 110 can generatethe K data streams 105 from return signals received from the gatewayterminals 130.

In some implementations, the beamforming of user beams by the forwardand/or return beamformers 110 is adaptive. For example, feedback is usedto adaptively compute the forward and/or return beam weights over time,thereby adapting the user beam beamforming to the feedback. Suchadaptation can tend to compensate for various types of non-idealities,such as changes in satellite attitude, changes in satellite reflectors(e.g., flexing, etc.), and/or other sources of beam pointing error. Incertain implementations, the user beam beamforming is non-adaptive. Forexample, where satellite attitude is well-controlled, and spatialdistribution of desired user beams is known, pre-calculated beamweightscan be applied by the forward and/or return beamformers 110 to form thedesired user beams. Some such implementations can be fully non-adaptive,while other such implementations can be partially adaptive (e.g., someadaptive loops can be used to address certain non-idealities, asdesired). In various implementations, adaptive and non-adaptivebeamforming can be used to support fixed or dynamic user beam generation(e.g., fixed or dynamic user beam sizes, user beam locations, and/orother user beam characteristics). For example, an adaptive fixedapproach can use adaptive beamforming to maintain user beam locations incontext of changing satellite attitude, while using fixed beamlocations. An adaptive dynamic approach can also change beam sizesand/or locations in response to changes in traffic demand, and anon-adaptive dynamic approach can cycle through pre-computed beamweights in different time slots to generate user beams of differentsizes and/or locations.

The gateway terminals 130 are also in communication with asynchronization subsystem 125. Embodiments of the synchronizationsubsystem 125 can mutually phase-synchronize uplink signals from thegateway terminals 130 so that the uplink signals will be received by thesatellite 140 in a phase-synchronous manner. For example, thesynchronization subsystem 125 can synchronize the carrier phase of theuplink signals from each gateway terminal 130 to account for path delaydifferences between each gateway terminal 130 and the satellite 140(e.g., the geographic distribution of the gateway terminals 130 yieldsdifferent distances between each gateway terminal 130 and thesatellite). Some embodiments can further time-synchronize the signals,for example, to line up symbol boundaries among transmissions from thedifferent gateway terminals 130. For example, this can help supportdynamic changes to modulation and/or encoding of data streams (e.g.,changes in modcodes), which can affect symbol durations and/or otherparameters.

The synchronization subsystem 125 can be implemented in any suitablemanner. In some implementations, each gateway terminal 130 includes, oris coupled with, a local instance of the synchronization subsystem 125.In other implementations, some or all gateway terminals 130 can share aninstance (e.g., a single, centralized instance) of the synchronizationsubsystem 125. For example, the shared synchronization subsystem 125 cansynchronize itself (or a single gateway terminal 130) with the satellite140, and can further synchronize the multiple other sharing gatewayterminals 130, accordingly. As described more fully below, varioustechniques can be used to perform various types of synchronization withthe synchronization subsystem 125. For example, the satellite 140 cantransmit a beacon signal that can be received by the synchronizationsubsystem 125 (via some or all gateway terminals 130); and thesynchronization subsystem 125 (e.g., each instance of thesynchronization subsystem 125 at each gateway terminal 130) can transmita loopback beacon signal. The synchronization subsystem 125 cansynchronize the satellite beacon and the loopback beacons to enablemutual phase-synchronization of the feeder-link signals.

FIG. 2 shows an illustrative satellite communications system 200 forimplementing forward-link communications, according to variousembodiments. The satellite communications system 200 can be animplementation of the satellite communications system 100 described withreference to FIG. 1. As illustrated, the satellite 140 providescommunications between a number (M) of geographically distributedgateway terminals 130 and a number of user terminals 165 in at least oneuser beam coverage area (corresponding to forward user beams 260). Thesatellite 140 includes a feeder antenna subsystem 230 and a user antennasubsystem 250. The feeder antenna subsystem 230 includes a number offocused-beam antenna elements (FAEs) 243, each illuminating a respectivefocused feeder beam. The feeder-link can operate with a single gatewayper beam, a group of gateways per beam, and/or in any other suitablemanner that enables bandwidth reuse through spatial multiplexing.Further, a single beam can be implemented using multiple antennaelements in some cases. The user antenna subsystem 250 includes an arrayof beamforming antenna elements (BAEs) 247 that can form one or moreuser beams (e.g., forward user beams 260) for communicating with userterminals 165 disposed in the coverage areas of those user beams. Notethat, because the system is a ground-based beamforming system, nophasing components need be included in the BAEs, as the phasing ofsignals transmitted by the BAEs is controlled by the phase relationshipsof the signals received by the corresponding FAEs as explained furtherbelow. The feeder antenna subsystem 230 and the user antenna subsystem250 can be implemented in various ways. In one implementation, thefeeder antenna subsystem 230 and/or the user antenna subsystem 250 isimplemented as a direct radiating array (e.g., the user antennasubsystem 250 can include a two-meter direct radiating array). Inanother implementation, the feeder antenna subsystem 230 and/or the userantenna subsystem 250 is implemented with separate transmit and receiveantennas. Some implementations also include one or more reflectors(e.g., an array fed reflector). One such implementation includes asingle reflector positioned, so that feeder feeds of the feeder antennasubsystem 230 are in focus with respect to the reflector, and user feedsof the user antenna subsystem 250 are out of focus with respect to thereflector. Another such implementation includes separate user and feederreflectors, for example, implemented so that feeder feeds of the feederantenna subsystem 230 are in focus with respect to the feeder reflector,and user feeds of the user antenna subsystem 250 are out of focus withrespect to the user. In one implementation, the one or more reflectorsare be implemented as di-chroic reflectors. For example, the di-chroicreflector(s) can include a sub-reflector that affects only uplinksignals, while higher-frequency downlink signals pass through thesurface of the sub-reflector.

The satellite further includes a forward repeater subsystem 240 having anumber (L) of forward-link pathways 245, where L equals the number ofgateway terminals 130 (M) (although, as explained below, in someembodiments, M can be less than L). There are many ways that the feederantenna system 230 may be connected to the forward repeater subsystem240. For example, in a case where the number of gateway terminals 130(M) is equal to the number of FAEs 243 (L), each FAE 243 can include oneforward-link output, each coupled with an input side of a respective oneof the forward-link pathways 245. Each BAE 247 can include aforward-link input coupled with an output side of a respective one ofthe forward-link pathways 245. In some cases, one or more FAEs 243 caninclude multiple forward-link outputs, each coupled with an input sideof a respective one of the forward-link pathways 245. The forwardrepeater subsystem 240 can have a forward uplink frequency range and aforward downlink frequency range. In some cases, the forward uplinkfrequency range overlaps (e.g., is partly or completely coextensivewith) the return uplink frequency range, and the forward downlinkfrequency range overlaps the return downlink frequency range (e.g.,feeder and user uplinks share a first frequency band and/or range, andfeeder and user downlinks share a second frequency band and/or range).For example, forward uplink signals can be received by a FAE 243 at theforward uplink frequency range, converted by the coupled forward-linkpathway 245 to the forward downlink frequency range, and transmitted bythe coupled BAE 247. As described below, this can enable forwarddownlink signals to be generable from beam-weighted, mutuallyphase-synchronized forward uplink signals that are each received at arespective one of the forward-link FAE inputs from a corresponding oneof the geographically distributed gateway terminals 130, such thattransmission of the forward downlink signals by the forward-link BAEoutputs causes the forward downlink signals to spatially superpose toform the at least one forward user beam 260.

As illustrated, each of K forward data streams 205 (e.g., fromfeeder-link modems, or the like) includes data destined for a respectiveone of the K forward user beams 260. In general, it is desired todistribute a beamformed version of those forward data streams 205 tosome or all of the M spatially separated gateway terminals 130. In thisway, the gateway terminals 130 can provide spatial multiplexing andbandwidth reuse for the forward communications. The forward data streams205 can be received by a forward beamformer 210 (e.g., part of theforward/return beamformers 110 of FIG. 1). The forward beamformer 210can apply L×K forward beam weights 213 to the forward data streams 205to generate L beam-weighted forward signals 215. For example, theforward beamformer 210 includes a forward data stream input, a beamweight input indicating a beam weight 213 associated with each of thegateway terminals 130; and beam-weighted forward signal outputs. Eachbeam-weighted forward signal output can be coupled with thebeam-weighted forward signal input of a respective one of the gatewayterminals 130 via the distribution network 120, and each can be aversion of the forward data stream input that has been beam-weightedaccording to the beam weight 213 associated with the respective one ofthe gateway terminals 130. Each of the L beam-weighted forward signals215 is generated to correspond to a respective one of the L forward-linkpathways 245 (and, accordingly, to a respective one of the L BAEs 247).The forward beam weights 213 are computed so that, when the weightedsignals are ultimately transmitted from the user antenna subsystem 250,the signals will spatially combine to form the forward user beams 260.

The forward beam weights 213 can be computed and applied in any suitablemanner. In some cases, the forward beam weights 213 are stored in aforward beam weight store of the forward beamformer 210. In other cases,a beam weight generator is part of, or coupled with, the forwardbeamformer 210. The forward beam weights 213 can be pre-computed, priorto deploying the satellite 140, based on simulated communication linkcharacteristics; computed one or more times (e.g., periodically) basedon feedback and analysis of the operating satellite communicationssystem 200; adjusted adaptively based on feedback and analysis of theoperating satellite communications system 200; and/or computed in anyother suitable manner. Many techniques are known for generating theforward beamforming coefficients. For example, the coefficients formultiple beams can be globally optimized to maximize the sum of thesignal to interference and noise ratios for all the beams. For example,at low signal to noise ratio, the weights may be chosen to maximizesignal power, while at high signal to noise ratio, the weights may beselected to minimize the intra beam interference.

Each of the L beam-weighted forward signals 215 can be communicated(e.g., via the distribution network 120) to a respective one of the Mgateway terminals 130. In many cases, the number of gateways terminals130 (M) is exactly the same as the number of forward link pathways 245(L). As will be discussed later, L may be greater than or equal to M, sothat each of the M gateway terminals 130 may receive one or more of theL beam-weighted forward signals 215 (corresponding to the one or moreforward-link pathways 245 coupled with each FAE 243, each FAE 243 beingassociated with a respective gateway terminal 130). Accordingly, the Mgateway terminals 130 can transmit the beam-weighted forward signals 215to the satellite 140 as L forward uplink signals 235. Prior totransmitting the forward uplink signals 235, the signals are mutuallyphase-synchronized. As described above, the gateway terminals 130include, or are in communication with, a synchronization subsystem 125that can mutually phase-synchronize the beam-weighted forward signals215 to generate the forward uplink signals 235. The mutualphase-synchronization causes the gateway terminals 130 to transmit thebeam-weighted forward signals 215 to the satellite 140 in such a waythat the forward uplink signals 235 are received in a phase-synchronousmanner by the FAEs 243 of the feeder antenna subsystem 230. For example,the synchronization accounts for path delay differences between eachgateway terminal 130 and the satellite 140, so that the signals receivedby the FAEs 243 have their carrier phases lined up and are at leastapproximately time-synchronized (e.g., to within a fraction of a desiredcommunication signal symbol period).

Thus, the satellite 140 receives multiple (L) beam-weighted, mutuallysynchronized forward uplink signals 235, each via a focused feederuplink (e.g., corresponding to one of the M gateway terminal 130locations). The satellite 140 can generate each of multiplebeam-weighted, mutually synchronized forward downlink signals 255 from acorresponding one of the plurality of forward uplink signals 235. Thesatellite 140 can transmit the forward downlink signals 255 viade-focused user downlinks, such that the forward downlink signals 255spatially superpose to form the one or more forward user beams 260. Forexample, each forward uplink signal 235 can be received by one of theFAEs 243 and passed to a coupled one of the forward-link pathways 245,which can generate a respective one of the forward downlink signals 255therefrom (e.g., by amplifying and frequency-converting the forwarduplink signals 235). Each forward downlink signal 255 can be passed to acoupled one of the BAEs 247 of the user antenna subsystem 250. The BAEs247 can transmit the forward downlink signals 255, and the beamweighting (e.g., and mutual synchronization) of the forward downlinksignals 255 causes them to spatially combine to form the forward userbeams 260. Each of the L BAEs 247 effectively transmits to each of Kuser beam coverage areas, so that L forward downlink signals 255 canspatially combine at each of the K user beam coverage areas to form arespective one of the K forward user beams 260.

FIG. 3 shows an illustrative satellite communications system 300 forimplementing return-link communications, according to variousembodiments. For the sake of clarity, the satellite communicationssystem 300 is illustrated to correspond to the satellite communicationssystems 200 described with reference to FIG. 2, and components arelabeled with similar or identical reference designators. As describedabove, the satellite 140 provides communications between a number ofuser terminals 165 in K user beam coverage area (corresponding to returnuser beams 360) and M geographically distributed gateway terminals 130.Some embodiments can use the same feeder antenna subsystem 230 and userantenna subsystem 250 in both the forward and return directions. Forexample, in the return direction, return uplink signals 355 can bereceived by the BAEs 247 of the user antenna subsystem 250 (from userterminals 165 in some or all of the return user beams 360). A returnrepeater subsystem 340, having return-link pathways 345 coupled withreturn-link outputs of the BAEs 247, can generate return downlinksignals 335 from the return uplink signals 355. FAEs 243 of the feederantenna subsystem 230, each having one or more return-link inputscoupled with respective ones of the return-link pathways 345, cantransmit the return downlink signals 335 to respective ones of thegateway terminals 130.

In some embodiments K return uplink signals 355 are received by each ofthe L BAEs 247. Each of the L BAEs 247 is coupled with a respective oneof L return-link pathways 345, so that L return downlink signals 335 aregenerated, each potentially including information transmitted from the Kreturn user beams 360. There are many ways that the return repeatersubsystem may be connected to the feeder antenna subsystem. For example,in a case where the number of gateway terminals 130 (M) is equal to thenumber of FAEs 243 (L), each FAE 243 can include one return-link input,each coupled with an output side of a respective one of the return-linkpathways 345. In some cases, one or more FAEs 243 can include multiplereturn-link inputs, each coupled with an output side of a respective oneof the return-link pathways 345 (e.g., as described in further detailbelow, in reference to FIG. 10). Thus, each of M FAEs 243 is coupledwith one or more of the L return-link pathways 345, so that the L returndownlink signals 335 are transmitted to the M gateway terminals 130. Thereceived return downlink signals 335 are neither beam-weighted normutually synchronized. The synchronization subsystem 125 can mutuallysynchronize the return downlink signals 335 (e.g., line up carrierphase, carrier timing, symbol boundaries, etc.) to generate L returnsignals 315 that are communicated to a return beamformer 310 (e.g.,either before or after transport via the distribution network 120). Thereturn signals 315 can be mutually synchronized from the perspective ofthe return beamformer 310. For example, in some embodiments, thesatellite 140 can transmit a beacon signal (e.g., a shared beacon) whichis received by each gateway terminal 130 and used to mutuallyphase-synchronize the return downlink signals 335 at the gateways 130relative to the beacon. The return beamformer 310 can apply L×K returnbeam weights 313 to the return signals 315 in such a way as to form thereturn user beams 360 (e.g., applying the return beam weights 313recovers the K return uplink signals via K retroactively formed returnuser beams 360). The return beam weights 313 can be computed and/orapplied in any suitable manner, for example, as described above withreference to computing and applying the forward beam weights 213 in FIG.2. In the return direction, however, intrabeam interference is generatedby the transmitting terminals, not by the beamforming process. Eachbeam's beamforming weights can thus be optimized individually. Byapplying the L×K return beam weights 313 to the synchronized L returnsignals 315, the return beamformer 310 can generate K return datastreams 305. In some embodiments, the return data streams 305 can bedemodulated by feeder-link modems, or the like, for communication via adigital data network, such as the Internet.

FIG. 4 shows a block diagram of a portion of an illustrative satellitesystem 400 for providing MSSMFL, according to various embodiments. Thesatellite system 400 includes multiple FAEs 243, each having aforward-link input and a return link output, which can collectively bereferred to as the feeder antenna subsystem. In some embodiments, thefeeder antenna system can also include one or more feeder reflectors410. For example, the feeder reflector(s) 410 can be used in conjunctionwith the FAEs 243 to focus feeder beams by which to communicate withgateway terminal locations. In some embodiments, the FAEs 243 areimplemented as a single feed per beam (SFPB) architecture. According tosuch an architecture, the FAEs 243 include antenna feeds, and eachantenna feed corresponds to a single respective one of the feeder beams.Some implementations include large numbers of feeder beams, such thatusing a SFPB architecture involves a comparably large number of antennafeeds. Physical size limitations of the antenna feeds can limit thephysical density of the antenna feeds (i.e., how closely the feeds canbe placed in the feeder antenna subsystem, which can effectively limitthe number of gateway beams that can be supported by a SFPBarchitecture. In other embodiments, the FAEs 243 are implemented as amultiple feeds per beam (MFPB) architecture. According to such anarchitecture, each antenna feed can be coupled with a weighted set ofbeam signals, so that each antenna feed can effectively be shared acrossmultiple feeder beams (e.g., in clusters of three antenna feeds, sevenantenna feeds, etc.).

The satellite system 400 also includes multiple BAEs 247, each having aforward-link output and a return link input, which can collectively bereferred to as the user antenna subsystem. In some embodiments, the userantenna system can also include one or more user reflectors 430. Forexample, the user reflector(s) 430 can be used in conjunction with theBAEs 247 to form forward and return user beams associated with user beamcoverage areas as described herein. In some embodiments, a singlereflector may be used as both a user reflector 430 and a feederreflector 410. In some embodiments, the user antenna subsystem can bethe same as the feeder antenna subsystem.

The satellite system 400 can further include a forward repeatersubsystem 240 and a return repeater subsystem 340. The forward repeatersubsystem 240 can have a forward uplink frequency range and a forwarddownlink frequency range and a number of forward-link pathways, eachcoupled between one of the forward-link inputs and one of theforward-link outputs. As illustrated, each forward-link input of a FAE243 can be coupled with a low-noise amplifier (LNA) 423 operating at theforward uplink frequency range. Each LNA 423 can be coupled with aninput side of a forward frequency converter 425, which can convert thereceived, amplified forward uplink signal from the forward uplinkfrequency range to the forward downlink frequency range. An output sideof each forward frequency converter 425 can be coupled with a poweramplifier (PA) 427 operating at the forward downlink frequency range.Each PA 427 can be coupled with the forward-link output of a respectiveone of the BAEs 247. Each coupled LNA 423, forward frequency converter425, and PA 427 can collectively implement a forward-link pathway (e.g.,one of the forward-link pathways 245 of FIG. 2). As described above,phase-synchronously receiving the mutually phase-synchronized,beam-weighted forward uplink signals (generated as such by feeder-sideground network components) at the FAEs 243 can cause the forwarddownlink signals also to be phase-synchronized and beam-weighted, suchthat transmission of the forward downlink signals by the BAEs 247 causesforward user beams to form by spatial superposition of the forwarddownlink signals.

The return repeater subsystem 340 can have a return uplink frequencyrange and a return downlink frequency range and a number of return-linkpathways, each coupled between one of the return-link inputs and one ofthe return-link outputs. As illustrated, each return-link input of a BAE247 can be coupled with an LNA 423 operating at the return uplinkfrequency range (e.g., which may or may not overlap with the forwarduplink frequency range). Each LNA 423 can be coupled with an input sideof a return frequency converter 435, which can convert the received,amplified return uplink signal from the return uplink frequency range tothe return downlink frequency range. An output side of each returnfrequency converter 435 can be coupled with a PA 427 operating at thereturn downlink frequency range. Each PA 427 can be coupled with thereturn-link output of a respective one of the FAEs 243. Each coupled LNA423, return frequency converter 435, and PA 427 can collectivelyimplement a return-link pathway (e.g., one of the return-link pathways345 of FIG. 3). In some cases, the return-link pathways are implementedin substantially the same manner, and with substantially the samecomponents, as the forward-link pathways. For example, the forward-linkpathways and the return-link pathways can be bent-pipe pathways; theforward-link pathways and the return-link pathways can be cross-bandpathways (i.e., each having an input side at a first frequency band andan output side at a second frequency band); etc. As described above,ground-based beamforming of the return downlink signals received bygeographically distributed gateway terminals 135 can effectively causereturn user beams to form with respect to the transmitted return uplinksignals.

In embodiments that include separate user antenna and feeder antennasubsystems, each antenna subsystem can be configured (e.g., optimized)for certain characteristics. For example, some MSSMFL communicationssystems can have different user and feeder beam coverage areas, whichmay or may not overlap; some MSSMFL communications systems can have someor all gateway terminals 130 disposed in user beam coverage areas; whileother MSSMFL communications systems can have some or all gatewayterminals 130 disposed outside user beam coverage areas; etc. In theseand other types of implementations, the feeder and user antennasubsystems (e.g., the reflectors, antenna elements, etc.) can befocused, pointed, and/or otherwise configured to provide communicationsservices to the different coverage areas and/or to providecommunications services to coverage areas in different ways. Forexample, the feeder reflector 410 can be configured with a largeraperture size than that of the user reflector 430 to illuminate thegateway terminals 130 with smaller focused beams. Such a configurationcan, for example, facilitate deployment of some or all gateway terminals130 in a smaller geographic region (e.g., as opposed to distributing thegateway terminals 130 across a larger user beam coverage area), whilemaintaining sufficient separation for desired bandwidth reuse. Forexample, gateway terminals 130 located only within the continentalUnited States can be used to service user terminals spread over a largerportion of the Earth's surface (e.g., the approximately one-third of theEarth visible from a geostationary satellite).

Larger feeder reflectors 410 can tend to have more surface distortion,to be more susceptible to flexing (e.g., due to temperaturedifferentials across the reflector, or the like), and/or to otherwiseimpact provision of focused feeder beams. For example, a largeunfurlable reflector has surface facets and/or ribs arranged as aperiodic structure that can create side lobes in the feeder antennapattern (e.g., similar in appearance to grating lobes). These and/orother impairments in the feeder antenna pattern can cause cross-talkand/or other interference between gateway terminals 130, which can beaddressed in various ways. In some implementations, some or all of thegateway terminals 130 are placed in locations that avoid interferencebetween the gateway terminals 130, accounting for patterns ofimpairment. FIG. 12 shows a plot 1200 of a feeder-link antenna patternfor an illustrative feeder reflector (e.g., feeder reflector 410)plotted with respect to azimuth and elevation. As illustrated, sidelobes in the feeder-link antenna pattern caused by surface distortionsof the reflector manifest as regions of feeder-link antenna patternimpairment 1210. Embodiments can determine (e.g., estimate, measure,compute, etc.) such impairment, and gateway terminals 130 can be placedin locations 1220 that avoid the regions of antenna pattern impairment1210.

In other implementations, interference between the gateway terminals 130due to feeder antenna pattern impairments can be compensated using oneor more impairment compensation approaches. According to a firstcategory of approaches, inter-gateway crosstalk is measured andcompensated (e.g., canceled) on the ground. Crosstalk can be measured invarious ways. One approach uses loopback beacons (described below) toestimate inter-gateway crosstalk. Because each gateway terminal 130knows the contents of its transmitted loopback beacon signal, eachreceived loopback beacon signal (i.e., the loopback signal received bythe transmitting gateway terminal 140 after being received and repeatedby the satellite 140) can be compared to the corresponding transmittedloopback signal to detect and measure crosstalk. Such an approach may beparticularly effective when most of the distortion is on the uplink,rather than on the downlink.

Another approach uses calibration at the user terminals 165 to measurethe crosstalk. To calibrate the feeder downlink side, a user terminal165 can transmit a probe signal, such that an expected feeder signal inthe response is known. Received feeder signals can be compared to theexpected feeder signals to measure crosstalk. Such an approach can beparticularly effective when adaptation of the return user link is notneeded. Multiple user terminals 165 across one or more user coverageareas can be used to measure crosstalk at different locations and/orcorresponding to different feeder downlinks. To calibrate the feederuplink side, one or more user terminals 165 can be used to measure aresponse when a probe signal is transmitted by a single gateway terminal130. Such an approach can be particularly effective when adaptation ofthe forward user link is not needed. Yet another approach is to usecalibration hardware on the satellite 140 to measure crosstalk. For thefeeder downlink side, a test signal can be injected by the satellite 140into each FAE (e.g., one at a time), and the gateway terminals 130 canmeasure crosstalk in the response to the feeder downlink signals. Forthe feeder uplink side, a probe signal can be transmitted by a gatewayterminal 130, and the response at each FAE measure (e.g., one at a time)and the response reported (or looped back) to the gateways. Havingmeasured crosstalk using any of the above or other approaches, some orall of the measured crosstalk can be removed. Equalization and/orcancelation hardware can be used to remove the measured crosstalk. Insome embodiments, the equalization and/or cancelation hardware isimplemented in the beamformer (e.g., the forward and/or returnbeamformers 110 of FIG. 1).

FIG. 15 shows a block diagram of an illustrative feeder antenna patternimpairment compensation environment 1500 that includes an illustrativecrosstalk canceler 1510. The illustrative crosstalk canceler 1510 can beimplemented in the beamformer 110 (e.g., coupled to the forwardbeamformer, return beamformer, or crosstalk cancelers coupled to both).As described above, feeder antenna pattern impairment can causecrosstalk in the feed signals received by the gateway terminals 130and/or satellite 140. Those feed signals are communicated between thegateway terminals 130 and the forward and/or return beamformers 110 viathe distribution network 120 and are received at feed signal input ports1512 of the crosstalk canceler 1510. Embodiments of the crosstalkcanceler 1510 can compensate for the crosstalk and can outputcompensated feed signals via feed signal output ports 1514. Someimplementations of the crosstalk canceler 1510 can receive a crosstalkmatrix 1520 (H). The crosstalk matrix 1520 can be a computed (e.g.,estimated) feeder matrix. The vector of feed signals with crosstalk canbe represented as f, such that f=H×f₀ (where f₀ is the vector of theoriginal feed signals without crosstalk, and H is the crosstalk matrix).For example, if H=I (the identity matrix), there is no crosstalk.Accordingly, the cross-talk compensated feed signals can be computed as{circumflex over (f)}=H⁻¹×f=H⁻¹×(H×f₀)=f₀. Crosstalk compensation can beperformed on forward uplink signals 235 (e.g. after forwardbeamforming), return downlink signals 335 (e.g., before returnbeamforming), or both. For example, for crosstalk compensation of theforward link, the beam-weighted forward input signals generated by theforward beamformer can be provided to the feed signal input ports 1512,and the cross-talk compensated feed signals at feed signal output ports1514 can be provided to the gateways as the beamweighted forward inputsignals; for crosstalk compensation of the return link, the returnsignal output from the gateways can be provide to the feed signal inputports 1512, and the cross-talk compensated feed signals at feed signaloutput ports 1514 can be provided to the return beamformer as the returnsignal outputs. Alternatively, the order of crosstalk compensation andbeamforming can be reversed.

Another category of approaches to impairment compensation involveslimited beamforming. As described herein, feeder beams are producedusing focused antenna elements, and some embodiments of the focusedantenna elements are implemented according to a multiple feeds per beam(MFPB) architecture. In MFPB architectures, each antenna feed can beassociated with a weighted and combined set of multiple feed signals. Toform each weighted and combined set, multiple feed signals can be passedthrough phase shifters, summers, and/or other hardware that can adjustamplitude weights, phase, and/or other characteristics of the resultingcombined feed signal according to applied coefficients. The appliedcoefficients can be stored in a beamforming coefficient memory on thesatellite 140. The coefficients can be pre-computed and/or adaptivelyupdated (e.g., using feedback to the satellite from the gateways), sothat the resulting combined feed signals can generated statically oradaptively. Applying the coefficients effectively applies somebeamforming to the feeder signals, which can be used to compensate forimpairments in the feeder antenna pattern. For example, crosstalk can bepredetermined (e.g., estimated or pre-computed) or measured using any ofthe approaches described above. Rather than using equalization orcancelation to remove the measured crosstalk, coefficients in the MFPBarchitecture can be used to apply limited beamforming to at leastpartially correct for the antenna impairments causing the crosstalk(e.g., in a predefined or adaptive manner). Some embodiments can use ahybrid of the categories of approaches. For example, equalization orcancelation can be used to remove measured crosstalk on one of thefeeder uplink or downlink, and limited beamforming can be used to removecrosstalk on the other of the feeder uplink or downlink.

Effective implementation of MSSMFL can rely on proper mutualsynchronization of signals among the spatially separated feeder links. Anumber of approaches can be used to implement such mutualsynchronization, and those approaches can depend on characteristics(e.g., overlap, etc.) of the user and feeder coverage areas. Forexample, in the forward direction, the gateway terminals 130 transmitforward uplink signals to the satellite 140, which are received byantenna elements of the feeder antenna subsystem 230, communicatedthrough forward-link pathways of the forward repeater subsystem 240 tothe user antenna subsystem 250, and transmitted as forward downlinksignals by antenna elements of the user antenna subsystem 250. To enablethe transmitted forward downlink signals to spatially combine to formforward user beams, embodiments use feeder-side ground segmentcomponents to generate the feeder signals 135 as mutually synchronized,beam-weighted forward uplink signals. In general, the satellite 140(e.g., the various components illustrated in FIG. 4) can be a means forrelaying mutually phase-synchronized, beam-weighted uplink signals asphase-coherent downlink signals that spatially combine to form forwarduser beams. For example, the FAEs 243 can receive mutuallyphase-synchronized, beam-weighted uplink signals; and the signals can bepassed through the forward repeater subsystem 240 and transmitted by theBAEs 247 as phase-coherent downlink signals that spatially combine toform forward user beams. To enable receipt of the forward uplinksignals, embodiments include multiple, spatially separated means forcommunicating the mutually phase-synchronized, beam-weighted uplinksignals; and means for mutually phase-synchronizing the uplink signalsat the spatially separated means for communicating, so that the forwarduplink signals will be received phase-synchronously by the means forrelaying. For example, embodiments use ground components in conjunctionwith satellite signaling to mutually synchronize feeder-link signalsbetween geographically distributed gateway terminals 130 and thesatellite 140. Similarly, in the return direction, the user terminalstransmit return uplink signals to the satellite 140, which are receivedby antenna elements of the user antenna subsystem 250, communicatedthrough return-link pathways of the return repeater subsystem 340 to thefeeder antenna subsystem 230, and transmitted as return downlink signalsby antenna elements of the feeder antenna subsystem 230. Usingfeeder-side ground segment components to mutually synchronize andbeam-weight the received return downlink signals can effectively formthe return user beams from which the return uplink signals weretransmitted.

Some embodiments use a combination of satellite beacon signaling andloopback beacon signaling to enable mutual synchronization of thefeeder-link signals. FIG. 5 shows a block diagram of an illustrativeloopback signal path 500 for loopback beacon signaling, according tovarious embodiments. For example, each gateway terminal 130 can transmita loopback beacon signal to the satellite 140, which is relayed from thesatellite 140 back to the gateway terminal 130 via the loopback signalpath 500. For each gateway terminal 130, the time elapsed betweensending and receiving its loopback signal can be used to compute itsdistance from the satellite 140 or otherwise allow for phase delay inthe feeder link to be compensated for. Similar to the forward andreturn-link pathways described above, a loopback pathway can include aloopback frequency converter 525 coupled between an LNA 423 and a PA427. Unlike the forward and return-link pathways described above, boththe input and output sides of the loopback pathway 500 can be incommunication with a loopback antenna element (LAE) 510. In someimplementations, the LNA 423 (input side) of the loopback pathway iscoupled to the forward-link output of an FAE 243 (i.e., the LAE 510 canbe implemented as a FAEs 243), and the PA 427 (output side) of theloopback pathway is coupled to the return-link input of the same or adifferent FAE 243. In some implementations, the LNA 423 of the loopbackpathway is coupled to the forward-link output of an FAE 243, and the PA427 of the loopback pathway is coupled to the forward-link input of aBAE 247 (e.g., where some or all gateway terminals 130 are within beamcoverage areas associated with the user antenna subsystem 250). In someimplementations, a separate loopback antenna subsystem is used. Forexample, the LAE 510 can be implemented as a coverage area antenna(e.g., including a wide-area horn) to receive and/or transmit loopbacksignals from/to some or all of the gateway terminals 130.

FIG. 6 shows a block diagram of a satellite beacon subsystem 600,according to various embodiments. As illustrated, the satellite beaconsubsystem 600 can include a master reference oscillator 610, a beaconsignal generator 620, a pseudo-noise (PN) code generator 630, redundantPAs 627, and a coverage area horn 640. The beacon signal generator 620can combine clocking signaling based on the master reference oscillator610 with PN signaling (e.g., using a code based on pseudorandom noise orany other suitable code) generated by the PN code generator 630 togenerate a beacon signal. Such a beacon signal can effectively provide aprecise phase reference (e.g., used for phase synchronization) and atimecode (e.g., used for symbol synchronization). Any other suitablebeacon signaling can be generated. The beacon signal can be amplified byone or more redundant PAs 627 and transmitted via the coverage areaantenna 640. In some implementations, the coverage area antenna 640 caninclude a wide-area horn. In other implementations, the coverage areaantenna 640 can include components for forming a wide beam to illuminatesome or all of the gateway terminals 130. In some implementations, thecoverage area antenna 640 can also be used to transmit one or morerelayed beacon signals.

As described above, some embodiments measure and/or compensate forfeeder-link antenna pattern impairment arising, for example, from feederreflector 410 distortion. Some approaches for measuring the feeder-linkantenna pattern impairment can use calibration circuitry on thesatellite 140 that exploits the loopback circuitry of the satellite 140.FIG. 13 shows a block diagram of an illustrative calibration system 1300implemented on the satellite 140 to assist in making forward-linkmeasurements of feeder-link antenna pattern impairments. The calibrationsystem 1300 can include couplers 1310 and switch 1320, which caninterface with the loopback frequency converter 525 described withreference to FIG. 5. Using switch 1320, the loopback frequency converter525 can be timeshared (e.g., set to different modes during differenttimeslots of a frame) to support both mutual synchronization of thegateway terminals 130, as well as estimation of feeder-link antennapattern impairment. During gateway synchronization time slots, theswitch 1320 can be set to cause a received feeder uplink signal to beprovided to the loopback frequency converter 525. During feeder antennapattern impairment measurement time slots, the switch 1320 can be set toselect one of the feeder antenna element 243 outputs to be provided tothe loopback frequency converter 525. By cycling through each of thedifferent feeder antenna elements 243 during different time slots, thegateway terminals 130 can measure the amount of uplink crosstalk betweeneach of the FAEs 243. The couplers can be any of: a directional coupler,signal sampler, power divider, unequal power divider, and/or othersuitable components. In some embodiments, gateway loopback signals areused for both gateway synchronization and pattern distortion estimation.The gateway loopback signals may be transmitted simultaneously with usertraffic (e.g., gateway loopback signals are spread-spectrum-encoded), ormay be transmitted during dedicated time slots where no user traffic ispresent.

FIG. 14 shows a block diagram of an illustrative calibration system 1400implemented on the satellite 140 to assist in making return-linkmeasurements of feeder-link antenna pattern impairments. Operation canbe similar to the forward-link calibration discussed above withreference to FIG. 13. Loopback signals transmitted through the loopbacktransponder can be selectively switched into individual feeder antennafeeds using switches 1410. Crosstalk on the feeder downlink can bemeasured by the gateway terminals 130. Return-link calibration can beperformed during dedicated timeslots (with one of switches 1410 beingset to provide the output of the loopback transponder to an individualfeed). Alternatively, switches 1410 can be combiners, in which casereturn-link calibration may be performed simultaneously withtransmission of return user traffic through the satellite 140.

Use of the loopback and satellite beacon signals for mutualsynchronization is described further in context of FIGS. 7 and 8. FIG. 7shows an illustrative satellite communications system 700 for providingMSSMFL, according to various embodiments. A satellite 140 providescommunications between a number of geographically distributed gatewayterminals 130 (via feeder signals 135) and a number of user terminals(via user signals 150). The satellite 140 includes a feeder antennasubsystem 230, a user antenna subsystem 250, a forward repeatersubsystem 240, a return repeater subsystem 340, and a satellitesynchronization subsystem 710. Both the gateway terminals 130 and theuser terminals are disposed in a user beam coverage area 760 (e.g.,which can be associated with one or more formed user beams, as describedabove). Though shown as a single, contiguous geographic region, the userbeam coverage area 760 can be any suitable shape and/or size, may or maynot be contiguous, may be the same or different in the forward andreturn directions, etc. Further, the geographic density and positioningof the gateway and user beams (and/or terminals) can be dictatedaccording to reflector sizes, number of antenna elements, number ofpathways, and/or other characteristics of the satellite communicationssystem 700. For example, the feeder antenna subsystem 230 can include anumber of focused antenna elements and a relatively large feederreflector, so as to form relatively narrow, focused feeder beams (ascompared to the reflector and beams of the user antenna subsystem 250).The relatively narrow beams can facilitate bandwidth reuse with closerspacing between the gateway terminals 130 (i.e., feeder beams can bemore tightly packed into a geographic area without overlap).Alternatively, some or all of the gateway terminals 130 can bedistributed throughout the user beam coverage area 760.

In the forward and return directions, contours of the user beam coveragearea 760 (e.g., which may or may not be the same in both directions) aredefined by formed forward and return user beams, respectively. Asdescribed above, embodiments described herein form the user beams byproviding ground-based beamforming of spatially multiplexed signals viaspatially separated (geographically distributed) feeder links, and suchspatially multiplexed ground-based beamforming can involve mutualsynchronization of the feeder signals 135. Embodiments enable suchmutual synchronization by coordination between a satellitesynchronization subsystem 710 and one or more ground-basedsynchronization subsystems 125 (e.g., each gateway terminal 130 caninclude an instance of the ground-based synchronization subsystem 125).

In some embodiments, such coordination involves synchronization of thegateway terminals 130 according to loopback and satellite beaconsignaling. For example, some embodiments of the satellitesynchronization subsystem 710 include a loopback pathway (e.g., asdescribed with reference to FIG. 5) and/or a beacon transmitter (e.g.,as described with reference to FIG. 6). As described above, thesatellite 140 can generate and transmit a beacon signal that can bereceived by the ground-based synchronization subsystem(s) 125, and thereceived beacon signal can be used, for example, to synchronize thephase of the reference oscillators in each gateway terminal 130 to thecarrier phase of the satellite 140. Further, the ground-basedsynchronization subsystem(s) 125 can transmit and receive one or moreloopback signals (e.g., a loopback beacon) via one or more loopbackpathways in the satellite 140. The received loopback beacon signal canbe used, for example, for range finding between each gateway terminal130 and the satellite 140. Such range finding can enable mutualsynchronization across the gateway terminals 130, so that gatewayterminals 130 can adjust their uplink transmissions to bephase-synchronously received at the satellite 140.

While MSSMFL can take advantage of mutual phase-synchronization of thefeeder-link signals, some implementations are less concerned withprecise signal time-alignment. Generally, the carrier frequency isorders of magnitude faster than the data rate, and a small slip in timealignment typically will not have a noticeable impact on symbol timingprovided that phase synchronization is maintained. In other words,occasional cycle slips can be tolerated provided that mutual phaserelationships between the feeder-link signals is maintained. Forexample, if the carrier frequency is 50 GHz, and a typical round-triptime (between a gateway terminal 130 and the satellite 140) is 240milliseconds, approximately twelve billion carrier cycles can elapseduring a single round trip. As such, a slip in time alignment of even afew hundred carrier frequency cycles can be relatively insignificantwith respect to data rates of 100 Mbps or higher. Still, some rangefinding (e.g., using the loopback signaling) can be desirable tofacilitate certain functionality. For example, it can be desirable toensure that changes in modulation occur for all forward uplink signalsat approximately the same time (e.g., at a corresponding symbol boundaryacross the spatially multiplexed signals). Some embodiments can includefurther synchronization for symbol timing. Certain implementations canuse the satellite beacon signal and/or the loopback signals for symboltiming synchronization. However, because the data rate is typicallyappreciably slower (e.g., 100 Mbps as compared to 15-75 GHz for Ku-band,Ka-band, V-band, etc.), many other techniques can be used. For example,other implementations can use global positioning satellite (GPS) timinginformation, or other techniques to achieve timing synchronization.

As illustrated, in the example satellite communications system 700, allthe gateway terminals 130 may be disposed in the user beam coverage area760. In such embodiments, some implementations of the satellitesynchronization system 710 can exploit antenna elements of the userantenna subsystem 250 for downlink communications with the ground-basedsynchronization subsystem(s) 125 (e.g., via respective gateway terminals130). In one such implementation, the feeder antenna subsystem 230 andthe user antenna subsystem 250 both operate in a same frequency band(e.g., Ka-band), and the gateway terminals 130 can receive signals fromthe satellite synchronization system 710 via the user antenna subsystem250.

For example, each antenna element of the feeder antenna subsystem 230can receive a PN-coded loopback beacon signal from an associated gatewayterminal 130 (each gateway terminal 130 can have its own unique PNcode), and those loopback beacon signals can be transmitted by the userantenna subsystem 250 to the user beam cover areas 760 in which thegateway terminals 130 are located. Each gateway terminal 130 can receivesome or all of the transmitted loopback signals (e.g., originating fromsome or all of the gateway terminals 130) and can correlate the receivedsignals against its own PN code, thereby recovering its loopback beaconsignal. The satellite synchronization system 710 can synchronize thegateway terminal 130 by aligning the recovered loopback beacon signal(e.g., phase- and/or time-aligning) with a satellite synchronizationsignal also received from the satellite 140. For example, the satellite140 transmits a satellite synchronization signal encoded with a PN codenot used by any of the gateway terminals 130).

In some such implementations, multiple reference locations (e.g., userterminals) can be used to determine forward-link beam weights. Forexample, the reference locations can include locations at or near thecenter of each (some or all) of the user beam coverage areas 760.Multiple (e.g., all) loopback beacon signals originating from multiple(e.g., all) gateway terminals 130 can be received by each referencelocation, and each reference location can correlate the gateway-specificPN codes with the received signals to recover the loopback beaconsignals. As described herein, each of the recovered loopback beaconsignals has traversed a forward link, including a forward uplink from acorresponding one of the gateway terminals 130 to the satellite 140, aforward pathway through the satellite 140, and a forward downlink fromthe satellite 140 to the reference location. Accordingly, the referencelocations can use the recovered signals to compute forward beam weightsfor characterizing those forward links. The computed forward beamweights can be fed back to the forward beamformer (e.g., via thesatellite 140, and gateway terminals 130), and the forward beamformercan determine whether to update the forward beam weights, accordingly.

FIG. 8 shows another illustrative satellite communications system 800for providing MSSMFL, according to various embodiments. The satellitecommunications system 800 is similar to the satellite communicationssystem 700 of FIG. 7, except that the gateway terminals 130 are disposedoutside the user beam coverage area 760. In such embodiments, thesatellite synchronization system 710 may not be able to exploit antennaelements of the user antenna subsystem 250 for downlink communicationswith the ground-based synchronization subsystem(s) 125 (e.g., viarespective gateway terminals 130). Some embodiments of the satellitesynchronization system 710 can implement loopback pathways usingforward-link inputs and return-link outputs of the antenna elements ofthe feeder antenna subsystem 230, so that the loopback beacon signalsare communicated in both directions as feeder signals 135. In some suchembodiments, the ground-based synchronization subsystem(s) 125 cansubtract out other feeder signals 135 to facilitate receipt of thebeacon signals. PN codes and/or other techniques can be used todifferentiate the synchronization signals from other feeder signals 135.For example, the PN-coded signal can be a relatively low-level signal(e.g., within or near the noise floor), and correlating the receivedsignal with the known PN code and sufficient gain (e.g., 25-30 dB), canenable recovery of the signal.

Other embodiments of the satellite synchronization system 710 caninclude a dedicated loopback antenna subsystem. For example, theloopback antenna subsystem can include one or more broad-beam antennasto cover the entire region (or regions) in which gateway terminals 130are disposed. In some such embodiments, where gateway terminals 130 arelocated outside of user beam coverage areas 760, the gateway terminals130 and user terminals can operate in the same band or in differentbands. For example, the gateway terminals 130 can communicate in theV-band, and the user terminals can communicate in the Ka-band.

FIGS. 9 and 10 illustrate techniques for reducing the number of gatewayterminals 130 for a MSSMFL deployment. FIG. 9 shows a block diagram ofan illustrative satellite system 900 for providing MSSMFL using multiplesignal polarizations, according to various embodiments, e.g., in a casewith distinct user and gateway coverage areas such as illustrated inFIG. 8. The satellite system 900 can be similar to the satellite system400 described with reference to FIG. 4, except that each of the multipleFAEs 243 (only one is shown for clarity) has multiple (e.g., two)forward-link inputs and multiple (e.g., two) return-link outputs. Eachforward-link input and each return-link output operates in a particularpolarization orientation. At the user antenna side, the BAEs 247 caneach still have a single forward-link output and return-link input. Forexample, all forward-link outputs of all the BAEs 247 can operate at afirst polarization orientation, while all return-link inputs of all theBAEs 247 can operate at a second polarization orientation. Thepolarization orientations used by the FAEs 243 can be the same as, ordifferent from, those used by the BAEs 247. For example, the FAEs 243can operate at left-hand circular polarization (LHCP) and right-handcircular polarization (RHCP), while the BAEs 247 can operate at linear(e.g., vertical and horizontal) polarizations. In cases where the FAEs243 and the BAEs 247 both use circular polarization orientations (orwhere both use linear polarizations, etc.), interference can arisebetween the feeder-link and user-link communications. For example, inthe illustrated implementation, LHCP is being used both for forwarddownlink signals and for some return downlink signals, and RHCP is beingused both for forward uplink signals and for some return uplink signals.Such potential interference can be mitigated or avoided by locatinggateway terminals 130 outside of user beams and/or by using variousinterference mitigation techniques (e.g., time or frequencymultiplexing). Further, as described above, signals are passed betweenthe FAEs 243 and the BAEs 247 via a forward repeater subsystem 240having forward-link pathways, and via a return repeater subsystem 240having return-link pathways. The forward-link and return-link pathwayscan be implemented in any suitable manner. As illustrated, each includesan LNA 423 at its input side, a PA 427 at its output side, and afrequency converter (425, 435) coupled between its LNA 423 and PA 427.Other implementations can include additional and/or alternativecomponents.

In some embodiments, each FAE 243 has a first forward-link input at afirst polarization orientation (e.g., LHCP), and a second forward-linkinput at a second polarization orientation (RHCP). The forward repeatersubsystem 240 includes a first number of forward-link pathways that areeach coupled between one of the first forward-link inputs and one of theforward-link outputs of a BAE 247 (e.g., operating at LHCP), and asecond number of forward-link pathways that are each coupled between oneof the second forward-link inputs and another of the forward-linkoutputs (e.g., also operating at LHCP). For example, the first and/orthe second number of forward-link pathways can convert from thepolarization orientation of the respective forward-link input to thedifferent polarization orientation of the respective forward-linkoutput. Similarly, each FAE 243 has a first return-link output at afirst polarization orientation (e.g., LHCP) and a second return-linkoutput at a second polarization orientation (e.g., RHCP). The returnrepeater subsystem 340 includes a first number of return-link pathwaysthat are each coupled between one of the return-link inputs of a BAE 247(e.g., operating at RHCP) and one of the first return-link outputs, anda second number of return-link pathways that are each coupled betweenone of the second return-link outputs and another of the return-linkinputs (e.g., also operating at RHCP). Again, the first and/or thesecond number of return-link pathways can convert from the polarizationorientation of the respective return-link input to the differentpolarization orientation of the respective return-link output.

In embodiments like the satellite system 900, each gateway terminal 130can communicate with the satellite 140 using any suitable number (e.g.,two) of orthogonal polarization orientations. Concurrent communicationsof feeder signals on multiple orthogonal polarization orientations caneffectively enable each gateway terminal 130 to reuse the entirety ofits allotted bandwidth in each polarization orientation withoutinterference. For example, such techniques can facilitate deployment ofhigher capacity satellite communications systems with fewer gatewayterminals 130. In such cases, the gateways can still cooperate tomutually synchronize their signals so that beamforming occurs.

FIG. 10 shows a block diagram of an illustrative satellite system 1000for providing MSSMFL using multiple frequency sub-ranges, according tovarious embodiments. The satellite system 1000 can be similar to thesatellite system 400 described with reference to FIG. 4, except that theforward feeder frequency range (received from each FAE 243) is separatedinto smaller forward user frequency sub-ranges for communication torespective coupled BAEs 247, and return user frequency sub-ranges(received from its coupled BAEs 247) are combined into a larger returnfeeder frequency range for communication to a respective coupled FAE243. In some embodiments, each forward-link input of an FAE 243 iscoupled with a frequency separator 1010 having a first frequencysub-range output and a second frequency sub-range output. The forwardrepeater subsystem 240 has a first number of forward-link pathways thatare each coupled between the first frequency sub-range output of one ofthe frequency separators 1010 and one of the forward-link outputs (e.g.,of a first BAE 247 operating in the first frequency sub-range), and asecond number of forward-link pathways that are each coupled between thesecond frequency sub-range output of one of the frequency separators1010 and another of the forward-link outputs (e.g., of a second BAE 247operating in the second frequency sub-range). Similarly, eachreturn-link output is coupled with a frequency combiner 1020 having afirst frequency sub-range input and a second frequency sub-range input.The return repeater subsystem 340 has a number of return-link pathwaysthat are each coupled between one of the return-link inputs (e.g., of afirst BAE 247 operating in the first frequency) and the first frequencysub-range input of one of the frequency combiners 1020, and a secondnumber of return-link pathways that are each coupled between another ofthe return-link inputs (e.g., of a second BAE 247 operating in thesecond frequency sub-range) and the second frequency sub-range input ofone of the frequency combiners 1020.

In the illustrated implementation each frequency separator 1010 iscoupled with the forward-link input of a respective FAE 243 via arespective LNA 423. In such an implementation, each forward-link pathwaycan be considered as including the LNA 423, a respective path throughthe frequency separator 1010, the forward frequency converter 425, andthe PA 427. In each forward-link pathway, the respective forwardfrequency converter 425 is coupled with a different output of thefrequency separator 1010 and converts a respective frequency sub-range.For example, each of forward frequency converter 425 a and forwardfrequency converter 425 b can include components that are selectedand/or adapted to tune its frequency conversion to its respectivefrequency sub-range. Similarly, in the illustrated implementation, eachfrequency combiner 1020 is coupled with the return-link output of arespective FAE 243 via a respective PA 427. In such an implementation,each return-link pathway can be considered as including the LNA 423, thereturn frequency converter 435, a respective path through the frequencycombiner 1020, and the PA 427. In each return-link pathway, therespective return frequency converter 435 is coupled with a differentoutput of the frequency combiner 1020 and converts a respectivefrequency sub-range. For example, each of return frequency converter 435a and return frequency converter 435 b can include components that areselected and/or adapted to tune its frequency conversion to itsrespective frequency sub-range.

In the illustrated embodiment, LHCP is used for forward uplink signalsand forward downlink signals, and RHCP is used for return uplink signalsand return downlink signals. This and other polarization schemes can beused to avoid interference between user-link and feeder-linkcommunications. In other implementations, other types of interferencemitigation can be used. For example, in one embodiment, a firstpolarization orientation is used by both forward and return uplinksignals, and a second polarization orientation is used by both forwardand return downlink signals; but gateway and user terminals communicateusing different frequencies and/or time slots.

In embodiments like the satellite system 1000, each gateway terminal 130can communicate with the satellite 140 using a feeder frequency rangethat encompasses any suitable number (e.g., two) of frequencysub-ranges. The sub-ranges may or may not be contiguous, overlapping,the same size, etc. Use of the frequency sub-ranges can enable eachgateway terminal 130 to feed multiple BAEs 247, thereby adding frequencymultiplexing to the spatial multiplexing of the gateway terminals 130.Using frequency multiplexing does result in some feeder bandwidthexpansion; for example, using two frequency sub-ranges result in twicethe bandwidth on the feeder link as on the user link. However, unlikeconventional GBBF, where the bandwidth expansion is proportional to thenumber of user antenna elements (e.g., hundreds), bandwidth expansionfor MSSMFL is proportional to the amount of frequency multiplexing(e.g., between one and ten). Stated another way, MSSMFL can allowtradeoffs between the feeder link bandwidth and number of gateways, anability not provided by conventional GBBF.

Techniques, such as those described with reference to FIGS. 9 and 10,can support a same amount of user-link bandwidth with fewer gatewayterminals 130. For example, suppose a satellite has L BAEs 247, and eachBAE 247 is allocated X GHz of user-link bandwidth, such that at leastL×X GHz of feeder-link bandwidth is needed to fully exploit theallocated user-link bandwidth. Embodiments of MSSMFL use mutuallysynchronized spatial multiplexing to enable GBBF with the L BAEs 247,while spreading the supporting L×X GHz of feeder-link bandwidth over Mgateway terminals 130. Where M=L, each gateway terminal 130 can beallocated X GHz of feeder-link bandwidth. As described in FIGS. 9 and10, some embodiments can use frequency and/or polarization multiplexingto permit a single gateway terminal 130 to support multiple BAEs 247,such that M can be less than L. For example, in some implementations,techniques described with reference to FIGS. 9 and 10 can be used inconjunction (e.g., multiple frequency sub-ranges within each of multiplepolarization orientations) to further reduce the number of gatewayterminals 130 needed to support a same amount of user-link bandwidth.

FIG. 11 shows a flow diagram of an illustrative method 1100 forground-based beamforming with MSSMFL in a satellite communicationssystem, according to various embodiments. The method 1100 begins atstage 1104 by receiving multiple beam-weighted, mutually synchronized(e.g., mutually phase-synchronized) forward uplink signals, eachreceived via a focused feeder uplink. For example, L feeder uplinks canbe received from M geographically distributed gateway terminals 130.Some embodiments of the method 1100 beam-weight a forward data stream togenerate L beam-weighted forward signals, and communicate each of the Lbeam-weighted forward signals to a corresponding one of M spatiallyseparated gateway locations. The beam-weighted forward signals can besynchronized (e.g., mutually phase-synchronized) at the gatewaylocations to generate L beam-weighted, mutually synchronized forwarduplink signals. According to some embodiments, the synchronizingincludes: receiving a beacon signal at the transmission locations, thebeacon signal generated by and transmitted from a relay (e.g., from awireless relay that performs the receiving at stage 1104); receiving, ateach transmission location, a respective loopback signal transmittedfrom the transmission location (e.g., and relayed back to thetransmission location via the relay); and phase-synchronizing, at eachtransmission location, the received respective loopback signal with thereceived beacon signal.

At stage 1108, embodiments generate each of multiple beam-weighted,mutually synchronized forward downlink signals from a corresponding oneof the forward uplink signals (e.g., via a forward repeater system of asatellite or other suitable relay). In some embodiments, the receivingis at an uplink frequency range, the transmitting is at a downlinkfrequency range, and the generating at stage 1108 includes convertingfrom the uplink frequency range to the downlink frequency range. Atstage 1112, embodiments transmit the forward downlink signals viamultiple de-focused user downlinks, such that the forward downlinksignals spatially superpose to form a user beam. In some embodiments,the forward downlink signals spatially superpose to form K user beams,and the beam-weighting includes applying L×K beam weights to K forwarddata streams to generate the L beam-weighted forward data signals.

Some embodiments continue at stage 1116 by receiving a multiple returnuplink signals via multiple de-focused user uplinks (e.g., from multipleuser terminals in one or more return user beam coverage areas). At stage1120, embodiments generate each of multiple return downlink signals froma corresponding one of the return uplink signals. At stage 1124,embodiments transmit the forward downlink signals via multiple spatiallyseparated focused feeder downlinks, the de-focused user uplinksoriginating in a beam coverage area formed by ground-based mutualphase-synchronizing and beam-weighting of the return downlink signalssubsequent to receiving the return downlink signals.

The above description provides various systems and methods that can beused to provide ground-based beamforming with mutually synchronized,spatially multiplexed feeder links. Some illustrative examples areprovided for added clarity. According to a first example, a satelliteincludes 400 elements and produces 200 beams. The uplink and downlinkcommunications are both in the Ka band (3.5 Gigahertz bandwidth). 200gateway terminals can be used with dual-polarization and no frequencymultiplexing on the feeder link, and single polarization can besupported on the user link. A spectral efficiency of 1.5 bits per secondper Hertz (bps/Hz) yields approximately 5 Gbps per beam (i.e., at 3.5Gigahertz per beam), and 5 Gbps/beam times 200 beams yieldsapproximately one Terabit per second of total capacity.

According to a second example, a satellite includes 512 elements andproduces 128 beams. The uplink and downlink communications are both inthe Ka band, with 2 Gigahertz of user-beam bandwidth and 4 Gigahertz offeeder-beam bandwidth. 128 gateway terminals can be used withdual-polarization and dual-frequency multiplexing on the feeder link,and single polarization can be supported on the user link. A spectralefficiency of 3 bps/Hz yields approximately 6 Gbps per user beam (i.e.,at 2 Gigahertz per user beam), and 6 Gbps/beam times 128 beams yieldsapproximately 768 Gbps of total capacity.

According to a third example, a satellite includes 768 elements andproduces 354 beams. The feeder link operates in V band with 7.5Gigahertz of feeder-beam bandwidth, and the user link operates in Kaband with 2.5 Gigahertz of user-beam bandwidth. 150 gateway terminalscan be used with dual-polarization and triple-frequency multiplexing onthe feeder link, and single polarization can be supported on the userlink. A spectral efficiency of 1 bps/Hz yields approximately 2.5 Gbpsper user beam (i.e., at 2.5 Gigahertz per user beam), and 2.5 Gbps/beamtimes 200 beams yields approximately 500 Gbps of total capacity.

According to a fourth example, a satellite includes 768 elements andproduces 354 beams. The feeder link operates in V band with 7 Gigahertzof feeder-beam bandwidth, and the user link operates in Ka band with 3.5Gigahertz of user-beam bandwidth. 192 gateway terminals can be used withdual-polarization and dual-frequency multiplexing on the feeder link,and single polarization can be supported on the user link. A spectralefficiency of 1.2 bps/Hz yields approximately 4 Gbps per user beam(i.e., at 3.5 Gigahertz per user beam), and 4 Gbps/beam times 354 beamsyields approximately 1.4 Tbps of total capacity.

The methods disclosed herein include one or more actions for achievingthe described method. The method and/or actions can be interchanged withone another without departing from the scope of the claims. In otherwords, unless a specific order of actions is specified, the order and/oruse of specific actions can be modified without departing from the scopeof the claims.

The functions described can be implemented in hardware, software,firmware, or any combination thereof. If implemented in software, thefunctions can be stored as one or more instructions on a tangiblecomputer-readable medium. A storage medium can be any available tangiblemedium that can be accessed by a computer. By way of example, and notlimitation, such computer-readable media can include RAM, ROM, EEPROM,CD-ROM, or other optical disk storage, magnetic disk storage, or othermagnetic storage devices, or any other tangible medium that can be usedto carry or store desired program code in the form of instructions ordata structures and that can be accessed by a computer. Disk and disc,as used herein, include compact disc (CD), laser disc, optical disc,digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disksusually reproduce data magnetically, while discs reproduce dataoptically with lasers.

A computer program product can perform certain operations presentedherein. For example, such a computer program product can be a computerreadable tangible medium having instructions tangibly stored (and/orencoded) thereon, the instructions being executable by one or moreprocessors to perform the operations described herein. The computerprogram product can include packaging material. Software or instructionscan also be transmitted over a transmission medium. For example,software can be transmitted from a website, server, or other remotesource using a transmission medium such as a coaxial cable, fiber opticcable, twisted pair, digital subscriber line (DSL), or wirelesstechnology such as infrared, radio, or microwave.

Further, modules and/or other appropriate means for performing themethods and techniques described herein can be downloaded and/orotherwise obtained by suitable terminals and/or coupled to servers, orthe like, to facilitate the transfer of means for performing the methodsdescribed herein. Alternatively, various methods described herein can beprovided via storage means (e.g., RAM, ROM, a physical storage mediumsuch as a CD or floppy disk, etc.), such that a user terminal and/orbase station can obtain the various methods upon coupling or providingthe storage means to the device. Moreover, any other suitable techniquefor providing the methods and techniques described herein to a devicecan be utilized. Features implementing functions can also be physicallylocated at various positions, including being distributed such thatportions of functions are implemented at different physical locations.

In describing the present invention, the following terminology will beused: The singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to an item includes reference to one or more items. The term“ones” refers to one, two, or more, and generally applies to theselection of some or all of a quantity. The term “plurality” refers totwo or more of an item. The term “about” means quantities, dimensions,sizes, formulations, parameters, shapes and other characteristics neednot be exact, but can be approximated and/or larger or smaller, asdesired, reflecting acceptable tolerances, conversion factors, roundingoff, measurement error and the like and other factors known to those ofskill in the art. The term “substantially” means that the recitedcharacteristic, parameter, or value need not be achieved exactly, butthat deviations or variations including, for example, tolerances,measurement error, measurement accuracy limitations and other factorsknown to those of skill in the art, can occur in amounts that do notpreclude the effect the characteristic was intended to provide.Numerical data can be expressed or presented herein in a range format.It is to be understood that such a range format is used merely forconvenience and brevity and thus should be interpreted flexibly toinclude not only the numerical values explicitly recited as the limitsof the range, but also interpreted to include all of the individualnumerical values or sub-ranges encompassed within that range as if eachnumerical value and sub-range is explicitly recited. As an illustration,a numerical range of “about 1 to 5” should be interpreted to include notonly the explicitly recited values of about 1 to about 5, but alsoinclude individual values and sub-ranges within the indicated range.Thus, included in this numerical range are individual values such as 2,3 and 4 and sub-ranges such as 1-3, 2-4 and 3-5, etc. This sameprinciple applies to ranges reciting only one numerical value (e.g.,“greater than about 1”) and should apply regardless of the breadth ofthe range or the characteristics being described. A plurality of itemscan be presented in a common list for convenience. However, these listsshould be construed as though each member of the list is individuallyidentified as a separate and unique member. Thus, no individual memberof such list should be construed as a de facto equivalent of any othermember of the same list solely based on their presentation in a commongroup without indications to the contrary. Furthermore, where the terms“and” and “or” are used in conjunction with a list of items, they are tobe interpreted broadly, in that any one or more of the listed items canbe used alone or in combination with other listed items. The term“alternatively” refers to selection of one of two or more alternatives,and is not intended to limit the selection to only those listedalternatives or to only one of the listed alternatives at a time, unlessthe context clearly indicates otherwise. The term “coupled” as usedherein does not require that the components be directly connected toeach other. Instead, the term is intended to also include configurationswith indirect connections where one or more other components can beincluded between coupled components. For example, such other componentscan include amplifiers, attenuators, isolators, directional couplers,redundancy switches, and the like. Also, as used herein, including inthe claims, “or” as used in a list of items prefaced by “at least oneof” indicates a disjunctive list such that, for example, a list of “atleast one of A, B, or C” means A or B or C or AB or AC or BC or ABC(i.e., A and B and C). Further, the term “exemplary” does not mean thatthe described example is preferred or better than other examples. Asused herein, a “set” of elements is intended to mean “one or more” ofthose elements, except where the set is explicitly required to have morethan one or explicitly permitted to be a null set.

Various changes, substitutions, and alterations to the techniquesdescribed herein can be made without departing from the technology ofthe teachings as defined by the appended claims. Moreover, the scope ofthe disclosure and claims is not limited to the particular aspects ofthe process, machine, manufacture, composition of matter, means,methods, and actions described above. Processes, machines, manufacture,compositions of matter, means, methods, or actions, presently existingor later to be developed, that perform substantially the same functionor achieve substantially the same result as the corresponding aspectsdescribed herein can be utilized. Accordingly, the appended claimsinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or actions.

What is claimed is:
 1. A satellite comprising: a feeder antennasubsystem comprising a plurality of focused-beam antenna elements(FAEs), each having a forward-link FAE port; a user antenna subsystemcomprising an array of beamforming antenna elements (BAEs), each havinga forward-link BAE port; and a forward repeater subsystem having aforward uplink frequency range and a forward downlink frequency range,the forward repeater subsystem comprising a plurality of forward-linkpathways, each of the plurality of forward-link pathways being coupledbetween one of the forward-link FAE ports and one of the forward-linkBAE ports, and each of the plurality of forward-link pathways having aninput in the forward uplink frequency range and an output in the forwarddownlink frequency range; such that a plurality of forward downlinksignals is generatable from beam-weighted, mutually phase-synchronizedforward uplink signals that are each received at a respective one of theFAEs from a corresponding one of a plurality of geographicallydistributed gateway terminals, the plurality of geographicallydistributed gateway terminals all to operate at a same carrierfrequency, and such that transmission of the forward downlink signals bythe BAEs causes the forward downlink signals to spatially superpose toform at least one forward user beam.
 2. The satellite of claim 1,further comprising: a satellite synchronization subsystem having abeacon transmitter, wherein the forward uplink signals are synchronizedprior to receipt at the FAEs in accordance with a synchronization signaltransmitted by the beacon transmitter.
 3. The satellite of claim 1,wherein: each FAE has a first forward-link FAE port at a firstpolarization orientation, and a second forward-link FAE port at a secondpolarization orientation; the plurality of forward-link pathwayscomprises: a first plurality of forward-link pathways, each coupledbetween one of the first forward-link FAE ports and one of theforward-link BAE ports; and a second plurality of forward-link pathways,each coupled between one of the second forward-link FAE ports andanother of the forward-link BAE ports.
 4. The satellite of claim 1,wherein: each forward-link input is coupled with a frequency separatorhaving a first frequency sub-range output and a second frequencysub-range output; and the plurality of forward-link pathways comprises:a first plurality of forward-link pathways, each coupled between thefirst frequency sub-range output of one of the frequency separators andone of the forward-link BAE ports; and a second plurality offorward-link pathways, each coupled between the second frequencysub-range output of one of the frequency separators and another of theforward-link BAE ports.
 5. The satellite of claim 1, further comprisinga return repeater subsystem having a return uplink frequency range and areturn downlink frequency range, wherein: each FAE of the feeder antennasubsystem further has a return-link FAE port; each BAE of the userantenna subsystem further has a return-link BAE port; and the returnrepeater subsystem comprises a plurality of return-link pathways, eachcoupled between one of the return-link BAE ports and one of thereturn-link FAE port, and each having an input in the return uplinkfrequency range and an output in the return downlink frequency range,such that a plurality of return downlink signals is generable by thereturn-link pathways for transmission by the FAEs, the return downlinksignals generable from return uplink signals received at the BAEs fromat least one user terminal, thereby forming at least one return userbeam for communications with the at least one user terminal.
 6. Thesatellite of claim 5, wherein: each FAE has a first return-link FAE portat a first polarization orientation and a second return-link FAE port ata second polarization orientation; each BAE has a first return-link BAEport at the first polarization orientation and a second return-link BAEport at the second polarization orientation; and the plurality ofreturn-link pathways comprises: a first plurality of return-linkpathways, each coupled between one of the first return-link BAE portsand one of the first return-link FAE ports; and a second plurality ofreturn-link pathways, each coupled between one of the second return-linkBAE ports and one of the second return-link FAE ports.
 7. The satelliteof claim 5, wherein: each return-link FAE port is coupled with afrequency combiner having a first frequency sub-range input and a secondfrequency sub-range input; and the plurality of return-link pathwayscomprises: a first plurality of return-link pathways, each coupledbetween one of the return-link BAE ports and the first frequencysub-range input of one of the frequency combiners; and a secondplurality of return-link pathways, each coupled between another of thereturn-link BAE ports and the second frequency sub-range input of one ofthe frequency combiners.
 8. The satellite of claim 5, wherein: theforward uplink frequency range overlaps the return uplink frequencyrange; and the forward downlink frequency range overlaps the returndownlink frequency range.
 9. The satellite of claim 1, wherein: thefeeder antenna subsystem further comprises a feeder reflector; and theuser antenna subsystem further comprises a user reflector.
 10. Asatellite communications system comprising: a plurality ofgeographically distributed gateway terminals, the plurality ofgeographically distributed gateway terminals all operating at a samecarrier frequency, and each comprising: a beam-weighted forward signalinput in communication with a forward beamformer via a distributionnetwork; a synchronization input coupled with a synchronizationsubsystem; and a feeder uplink signal output that corresponds to amutually synchronized version of the beam-weighted forward signal inputin accordance with the synchronization input; and a satellitecomprising: a feeder antenna subsystem comprising a plurality offocused-beam antenna elements (FAEs), each FAE having a forward-link FAEport that is communicatively coupled with the feeder uplink signaloutput of a corresponding one of the gateway terminals; a user antennasubsystem comprising an array of beamforming antenna elements (BAEs),each having a forward-link BAE port; and a forward repeater subsystemhaving a forward uplink frequency range and a forward downlink frequencyrange, the forward repeater subsystem comprising a plurality offorward-link pathways, each coupled between one of the forward-link FAEports and one of the forward-link BAE ports, and each having an input inthe forward uplink frequency range and an output in the forward downlinkfrequency range, such that a plurality of forward downlink signals isgenerated from forward uplink signals received at the FAEs, such thattransmission of the forward downlink signals by the BAEs causes theforward downlink signals to spatially superpose to form forward userbeams.
 11. The satellite communications system of claim 10, wherein:each FAE has a first forward-link FAE port at a first polarizationorientation, and a second forward-link FAE port at a second polarizationorientation; the plurality of forward-link pathways comprises: a firstplurality of forward-link pathways, each coupled between one of thefirst forward-link FAE ports and one of the forward-link BAE ports; anda second plurality of forward-link pathways, each coupled between one ofthe second forward-link FAE ports and another of the forward-link BAEports.
 12. The satellite communications system of claim 10, wherein:each forward-link input is coupled with a frequency separator having afirst frequency sub-range output and a second frequency sub-rangeoutput; and the plurality of forward-link pathways comprises: a firstplurality of forward-link pathways, each coupled between the firstfrequency sub-range output of one of the frequency separators and one ofthe forward-link BAE ports; and a second plurality of forward-linkpathways, each coupled between the second frequency sub-range output ofone of the frequency separators and another of the forward-link BAEports.
 13. The satellite communications system of claim 10, wherein:each geographically distributed gateway terminal further comprises: areturn signal output in communication with a return beamformer via thedistribution network; and a feeder downlink signal input; and each FAEof the feeder antenna subsystem further has a return-link FAE port; eachBAE of the user antenna subsystem further has a return-link BAE port;and the satellite further comprises a return repeater subsystem having areturn uplink frequency range and a return downlink frequency range,wherein the return repeater subsystem comprises a plurality ofreturn-link pathways, each coupled between one of the return-link BAEports and one of the return-link FAE ports, and each having an input inthe return uplink frequency range and an output in the return downlinkfrequency range, such that a plurality of return downlink signals isgenerable by the return-link pathways for transmission by the FAEs, thereturn downlink signals generable from return uplink signals received atthe BAEs from at least one user terminal, thereby forming at least onereturn user beam for communications with the at least one user terminal.14. The satellite communications system of claim 13, wherein: each FAEhas a first return-link FAE port at a first polarization orientation anda second return-link FAE port at a second polarization orientation; eachBAE has a first return-link BAE port at the first polarizationorientation and a second return-link BAE port at the second polarizationorientation; and the plurality of return-link pathways comprises: afirst plurality of return-link pathways, each coupled between one of thefirst return-link BAE ports and one of the first return-link FAE ports;and a second plurality of return-link pathways, each coupled between oneof the second return-link BAE ports and one of the second return-linkFAE ports.
 15. The satellite communications system of claim 13, wherein:each return-link FAE port is coupled with a frequency combiner having afirst frequency sub-range input and a second frequency sub-range input;and the plurality of return-link pathways comprises: a first pluralityof return-link pathways, each coupled between one of the return-link BAEports and the first frequency sub-range input of one of the frequencycombiners; and a second plurality of return-link pathways, each coupledbetween another of the return-link BAE ports and the second frequencysub-range input of one of the frequency combiners.
 16. The satellitecommunications system of claim 13, wherein: the forward uplink frequencyrange overlaps the return uplink frequency range; and the forwarddownlink frequency range overlaps the return downlink frequency range.17. The satellite communications system of claim 10, wherein: the feederantenna subsystem further comprises a feeder reflector; and the userantenna subsystem further comprises a user reflector.
 18. The satellitecommunications system of claim 17, wherein: surface distortions on thefeeder reflector define feeder antenna impairment regions; and theplurality of geographically distributed gateway terminals are positionedaway from the feeder antenna impairment regions.
 19. The satellitecommunications system of claim 17, wherein: surface distortions on thefeeder reflector define feeder antenna impairment regions; and thefeeder antenna subsystem comprises a multiple feeds per beam antenna toapply limited beamforming to feeder beams to compensate for the feederantenna impairment regions.
 20. The satellite communications system ofclaim 19, wherein: the multiple feeds per beam antenna comprises abeamforming coefficient memory having pre-computed beamformingcoefficients stored therein, the pre-computed beamforming coefficientsused to apply the limited beamforming.
 21. The satellite communicationssystem of claim 19, wherein: the multiple feeds per beam antennacomprises a beamforming coefficient memory having adaptively updatedcoefficients stored therein, the adaptively updated coefficients used toapply the limited beamforming.
 22. The satellite communications systemof claim 10, wherein: the plurality of geographically distributedgateway terminals comprises M gateway terminals; and the forwardrepeater subsystem comprises M forward-link pathways, each correspondingto a respective one of the M gateway terminals.
 23. The satellitecommunications system of claim 10, wherein the feeder uplink signaloutput corresponds to a phase-synchronized version of the beam-weightedforward signal input responsive to the synchronization input.
 24. Thesatellite communications system of claim 10, wherein: the satellitefurther comprises a beacon transmitter; and the synchronizationsubsystem comprises: a loopback input; and a synchronization outputcoupled with the synchronization input, the synchronization outputresponsive to phase-synchronization of a beacon signal and a loopbacksignal both received at the loopback input.
 25. The satellitecommunications system of claim 24, wherein each gateway terminal furthercomprises a loopback transmitter.
 26. The satellite communicationssystem of claim 10, wherein each gateway terminal further comprises alocal instance of the synchronization subsystem.
 27. The satellitecommunications system of claim 10, further comprising: thesynchronization subsystem, wherein each of the plurality ofgeographically distributed gateway terminals is coupled with thesynchronization subsystem via the distribution network.
 28. Thesatellite communications system of claim 10, further comprising: acrosstalk canceler having: a plurality of feed input ports to receivefeed input signals from the plurality of gateway terminals; and aplurality of feed output ports to transmit crosstalk-compensated feedsignals, the crosstalk-compensated feed signals generated as a functionof the feed input signals and a stored crosstalk matrix.
 29. Thesatellite communications system of claim 28, wherein the crosstalkcanceler is disposed in the beamformer.
 30. The satellite communicationssystem of claim 10, wherein: the satellite further comprises a loopbackpathway having a loopback antenna element that is coupled with the FAEsvia a plurality of switches, such that, over a plurality of time slots,the loopback antenna element is sequentially coupled with the respectiveforward-link FAE port of each of the FAEs, thereby, in each of the timeslots, transmitting a repeated loopback signal to the gateway terminalsin response to receiving a loopback beacon signal from the gatewayterminal associated with the FAE that is sequentially coupled in thetime slot.
 31. The satellite communications system of claim 13, wherein:the satellite further comprises a loopback pathway having a loopbackantenna element that is coupled with the FAEs via a plurality ofswitches, such that, over a plurality of time slots, the loopbackantenna element is sequentially coupled with the respective return-linkFAE port of each of the FAEs, thereby, in each of the time slots,transmitting a repeated loopback signal to the gateway terminalassociated with the FAE that is sequentially coupled in the time slot inresponse to receiving a loopback beacon signal from the gatewayterminals.
 32. The satellite communications system of claim 10, wherein:the feeder antenna subsystem illuminates a plurality of forward feederbeams; and the formed forward user beams do not spatially overlap withthe forward feeder beams.
 33. The satellite communications system ofclaim 10, further comprising: the forward beamformer, which comprises: aplurality of forward data stream inputs; a beam weight input indicatingbeam weights associated with each of the gateway terminals and forwarddata streams; and a plurality of beam-weighted forward signal outputs,each coupled with the beam-weighted forward signal input of a respectiveone of the gateway terminals via the distribution network, and eachbeing a weighted sum of the forward data stream inputs beam-weightedaccording to the beam weights associated with the respective one of thegateway terminals.
 34. The satellite communications system of claim 33,wherein: the forward data stream input comprises K forward data streaminputs, each corresponding to a respective one of K formed forward userbeams; the plurality of geographically distributed gateway terminalscomprises L feeder uplink signal outputs; and the beam weight inputcomprises L×K beam weights; and the beam-weighted forward signal outputscorrespond to L composites of the K forward data stream inputsbeam-weighted according to the L×K beam weights.
 35. The satellitecommunications system of claim 10, wherein the satellite is ageostationary satellite.
 36. A method for ground-based beamforming in asatellite communications system, the method comprising: receiving aplurality of beam-weighted mutually synchronized forward uplink signals,each received via one of a plurality of focused feeder uplink antennaelements from a corresponding one of a plurality of spatially separatedgateway locations, wherein the plurality of spatially separated gatewaylocations all operate at a same carrier frequency, the plurality offocused feeder uplink antenna elements being focused-beam antennaelement (FAE) of a feeder antenna system of a satellite; generating eachof a plurality of forward downlink signals by amplifying and frequencyconverting a corresponding one of the plurality of forward uplinksignals; and transmitting the plurality of forward downlink signals viaa plurality of de-focused user downlink antenna elements, such that theforward downlink signals spatially superpose to form a user beam, theplurality of de-focused user downlink antenna elements being beamformingantenna element (BAE) of a user antenna system of the satellite coupledwith the FAEs via forward-link pathways.
 37. The method of claim 36,further comprising: applying beam-weighting to a plurality of forwarddata stream to generate L beam-weighted forward signals; communicatingeach of the L beam-weighted forward signals to a corresponding one of Mspatially separated gateway locations; and synchronizing thebeam-weighted forward signals at the gateway locations to generate Lbeam-weighted, mutually synchronized forward uplink signals.
 38. Themethod of claim 37, wherein: the forward downlink signals spatiallysuperpose to form K user beams; and the beam-weighting comprisesapplying L×K beam weights to K forward data streams to generate the Lbeam-weighted forward signals.
 39. The method of claim 36, wherein: thereceiving is at an uplink frequency range; the transmitting is at adownlink frequency range; and the frequency converting comprisesconverting from the uplink frequency range to the downlink frequencyrange.
 40. The method of claim 36, further comprising: receiving aplurality of return uplink signals via a plurality of de-focused useruplink antenna elements; generating each of a plurality of returndownlink signals from a corresponding one of the plurality of returnuplink signals; and transmitting the plurality of return downlinksignals via a plurality of focused feeder downlink antenna elements tothe spatially separated gateway locations, the de-focused user uplinksoriginating in a beam coverage area formed by ground-based mutualphase-synchronizing and beam-weighting of the return downlink signalssubsequent to transmitting the return downlink signals.
 41. The methodof claim 40, further comprising: receiving the plurality of returndownlink signals at the spatially separated gateway locations;synchronizing the return downlink signals at the gateway locations togenerate a plurality of mutually synchronized return signals; andbeam-weighting the plurality of mutually synchronized return signals togenerate beam-weighted mutually synchronized return signals.
 42. Themethod of claim 41, wherein: the beam-weighted mutually synchronizedreturn signals comprise L beam-weighted mutually synchronized returnsignals; the formed beam coverage area comprises K user beams; andbeam-weighting the plurality of mutually synchronized return signalscomprises applying L×K return beam weights to the L beam-weightedmutually synchronized return signals to recover K return data streams.43. The method of claim 40, wherein: the plurality of return uplinksignals are received at a return uplink frequency range; the pluralityof return downlink signals are transmitted at a return downlinkfrequency range; and the generating comprises converting from the returnuplink frequency range to the return downlink frequency range.
 44. Themethod of claim 37, wherein: the receiving is by a wireless relay; andthe synchronizing comprises: receiving a beacon signal at the gatewaylocations, the beacon signal transmitted from the relay; receiving, ateach gateway location, a respective loopback signal transmitted from thegateway location; and phase-synchronizing, at each gateway location, thereceived respective loopback signal with the received beacon signal. 45.The method of claim 36, further comprising: receiving feed input signalsfrom the gateway locations; and generating crosstalk-corrected feedsignals as a function of the feed input signals and a stored crosstalkmatrix.
 46. The method of claim 45, further comprising: sequentiallycoupling a loopback antenna element, over a plurality of time slots,with each of the focused feeder uplink antenna elements, thereby, ineach of the time slots, transmitting a repeated loopback signal to thegateway locations in response to receiving a loopback beacon signal fromthe gateway location associated with the focused feeder uplink antennaelement that is sequentially coupled in the time slot, wherein, over theplurality of time slots, the feed input signals correspond to therepeated loopback signals as received at the gateway locations.
 47. Themethod of claim 45, further comprising: sequentially coupling a loopbackantenna element, over a plurality of time slots, with each of aplurality of focused feeder downlink antenna elements, thereby, in eachof the time slots, transmitting a repeated loopback signal to thegateway location associated with the focused feeder downlink antennaelements that is sequentially coupled in the time slot in response toreceiving a loopback beacon signal from the gateway locations, wherein,in each of the time slots, at least one of the feed input signalscorresponds to the repeated loopback signal as received by at least oneof the gateway locations in the time slot.
 48. The method of claim 36,further comprising: receiving at least one probe signal by at least oneof the gateway locations from at least one user terminal; and generatingat least one crosstalk-corrected feed signal as a function of comparingthe at least one probe signal with an expected feed signal.
 49. Themethod of claim 36, further comprising: receiving at least one probesignal by at least one user terminal from at least one of the gatewaylocations; and generating at least one crosstalk-corrected feed signalas a function of comparing the at least one probe signal with anexpected user signal.